The present invention relates to grinding systems, particularly those intended for use with angle grinders. Typical grinding discs comprise a substrate or backing material upon which is deposited a maker coat which is used to adhere a coating of abrasive particles applied to the maker coat before it is cured. A size coat is conventionally applied over the abrasive particles to ensure that they are firmly anchored. A supersize coat may be applied over the size coat to confer added properties such as anti-loading, lubrication, grinding aids and the like. More recently other grinding surfaces have been provided in which the abrasive particles are dispersed within a binder which is then deposited on the substrate such that the abrasive material is made in a single step. This binder/abrasive layer may be deposited in a continuous layer that may be smooth or engineered to have a profiled surface with spaced abrading points. Alternatively it may be deposited in isolated islands leaving a profiled surface which also provides spaced abrading points. Such profiled surfaces are very suitable for fine finishing and polishing especially when the particle are small, such as below about 150 microns in average particle size. Such grinding discs are supported on a backup pad which, together with the disc, forms the grinding system as the term is used in this invention.
The drawback of the traditional round abrading disc is that it is not possible to see the surface that is being ground such that it is necessary to grind and then remove to view the surface before grinding again and removing again to view the results. In addition the typical grinding process using conventional discs uses the disc with an attack angle to the workpiece surface of about 45 degrees. This results in gouging unless the operator is quite skilled. These problems were overcome in the invention described in PCT/US96/19191. The abrasive discs described in this Application comprise circular discs having portions removed from at least three spaced positions around the circumference of the disc and holes through the body of the disc, such that the combination of peripheral gaps and holes allow essentially complete view of the portion of the workpiece being ground as it is being ground. In addition to the increased vision and therefore control of the operation, the disc is designed to be used at a very much lower attack angle of about 15 degrees such that a much higher percentage of the actual disc surface is used. By contrast when operating at the traditional high angle of attack the disc has to be discarded after only the outer half inch or so of the periphery of the disc has been worn out. This translates to a much longer life for the disc along with cooler cutting. Such discs are designed to be carried on a backup pad with similar outline shapes. These are described in PCT/US96/18927.
The portions removed from the disc circumference according to the above specification are not restricted to straight chord segments but could include portions that leave the outer perimeter of the disc with a curved outline. The present invention relates to a particularly preferred outline that confers specific advantages especially when working on a surface that meets a second surface angled upward with respect to the surface being ground. In such situations it is possible for the edge of the disc to snag against the angled surface and perhaps tear the disc. The present invention represents a preferred solution to this situation that significantly reduces the consequences of a contact with such an angled surface.